System and method for locating radio frequency identification tags using three-phase antenna

ABSTRACT

A system and method for determining the position of a radio frequency identification (RFID) transponder with respect to a sensor. In one embodiment, the system comprises a plurality of stationary sensors located in an array within certain physical areas. Each sensor comprises a plurality of antenna coils arranged in unique physical orientations and capable of transmitting radio frequency signals of differing phase. The RFID transponder includes an antenna which receives the plurality of signals generated by the antenna coils, and compares the phase of at least two of the signals to determine the relative position of the transponder. In a second aspect of the invention, the aforementioned antenna coils emit two direction finding mode (DFM) signals in succession; the first signal with all antenna coils turned on, the second with one of the coils turned off. The spatial relationship of the transponder antenna and individual antenna coils precludes all of the signals in each sensor from being rejected by the transponder during emission of both the first and second DFM signal. Hence, the transponder is kept in constant communication with the sensor in all orientations. In another embodiment, the location of the transponder with respect to two or more sensor(s) is determined through measurement of the intensity of the signals received by the antenna coil of the transponder. The invention also includes a system and method for transmitting data between a sensor and a dormant (motionless) RFID transponder using a hand-held high intensity RF probe.

RELATED APPLICATIONS

This patent application is related to U.S. application Ser. No.09/405,358, entitled “SYSTEM AND METHOD FOR COMMUNICATION WITH RADIOFREQUENCY IDENTIFICATION TAGS USING TWO MESSAGE DFM PROTOCOL”; U.S.application Ser. No. 09/405,634, entitled “SYSTEM AND METHOD FORCOMMUNICATING WITH DORMANT RADIO FREQUENCY IDENTIFICATION TAGS”, andU.S. application Ser. No. 09/406,092, entitled “SYSTEM AND METHOD FORLOCATING RADIO FREQUENCY IDENTIFICATION TAGS”, which are being filedconcurrently herewith on Sep. 24, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to radio frequency communication, andspecifically to the use of radio frequency identification (RFID) tags inconjunction with one or more radio frequency sensors to determine theposition of the tag or an asset to which the tag is attached withinthree dimensional space.

2. Description of the Related Technology

The field of RFID (Radio Frequency Identification) is growing rapidly.Applications of RFID technology are wide ranging and include countingobjects as they pass near to a sensor, uniquely identifying a specifictag (hereinafter “transponder”) and associated asset, and placing datawithin the RFID transponder for later recovery. The process of “reading”and communicating with an RFID transponder generally comprises bringingthe transponder in proximity to an RFID sensor which emanates a radiofrequency wake-up field having a limited range. The RFID transponderdetects the presence of the wakeup field of the sensor, and subsequentlyvarious forms or protocols of handshake occur between the transponderand the sensor in order to exchange data. All of this communicationbetween the transponder and the sensor is performed using radiofrequency carriers of some kind. When multiple transponders areinvolved, anti-clash protocols of the type well understood in the dataprocessing arts are employed in order to multiplex or provide multipleaccess to the sensor by the multiple transponders.

The main advantages of an RFID sensor and transponder system over otherforms of ID tagging include (i) the orientation of the transponder withrespect to the sensor is not critical for a correct read of thetransponder information; (ii) communication can occur withincomparatively harsh operating environments including dirt, grease,opaque gasses, etc.; and (iii) the communication range between thesensor and transponder can be significant (in excess of 100 feet incertain cases) even when the RF frequencies used are within the powerlimitations of Federal Communications Commission (FCC) rules concerningunlicensed transmitters. Accordingly, RFID technology is useful forseveral applications, especially those relating to security and assetmanagement.

For example, in applications where enhanced security is desired, RFIDsystems using electromagnetic energy with very low frequency areattractive since the low frequency energy tends to suffer low lossesfrom shielding materials such as metal boxes, aluminum foil, and thelike. Those who would surreptitiously remove the tagged assets from abuilding usually try to use such shielding techniques. However, theselow frequencies typically require large antennas within the transponderin order to achieve reasonable levels of RF coupling between the sensorand the transponder. It is impractical to place large wire antennaswithin small transponders; accordingly, comparatively small magneticloop antennas are the coupling methods of choice for such smalltransponders. These magnetic loop antennas exhibit a serious drawback,however, in that they have a characteristic “figure-8” sensitivitypattern and, in certain positions and/or orientations, can reject orotherwise not detect the fields generated from the sensor. Stateddifferently, the magnetic loop antenna of the transponder can onlyreceive energy from the sensor antenna coils only when the orientationof the sensor and transponder coils is similar. Specifically, the“rejection” solid angle for a loop antenna can be thought of as a bandof a certain solid angle measured from the center and oriented 360degrees around the circumference of the loop (see FIG. 1). When suchrejection occurs, the transponder may be well within the sensor'sintended wake up field, but fails to detect the sensor's emissions, andtherefore also fails to communicate therewith. A related problem is whenthe position and/or orientation of the transponder within the field isvaried, thereby taking the sensor(s) out of the “figure-8” pattern ofthe transponder antenna, and interrupting communication between thetransponder and sensor.

Additionally, many existing RFID transponder/sensor systems do not havethe ability to locate the transponder in spatial space. Those which dohave this ability suffer from significant drawbacks since none of themwill function using the low frequency signals needed to pass throughfoil and other shielding. The added capability of spatial positioning,however, allows the sensor to gather more information about thetransponder, i.e., its relative location in space with respect to thesensor or some other reference point. This capability provides a verysignificant advantage over other asset management systems (RFID orotherwise) which can not determine the position of the assets.

Furthermore, existing RFID systems in which the transponder includes amotion sensor or other device which activates or otherwise permits thewaking up of the transponder do not have provision for the transponderto communicate with the system sensor (reader) during periods when thetransponder is not in motion, such as during installation ormaintenance. Accordingly, such prior art transponders must be physicallymoved or agitated during these periods in order to enable thetransponder to communicate with the sensor. This approach is cumbersomeand inefficient.

Based on the foregoing, an improved apparatus and method for spatiallylocating an RFID transponder having a magnetic loop antenna within oneor more sensor fields is needed. Furthermore, an improved apparatus andmethod for maintaining effectively constant and uninterruptedcommunication with the aforementioned RFID transponder regardless ofphysical position or orientation is needed. Lastly, an improvedapparatus for interrogating and communicating with the RFID transponderwhen the transponder is not in motion or otherwise dormant is needed.

SUMMARY OF THE INVENTION

The foregoing needs are addressed by the invention disclosed herein.

In a first aspect of the invention, an improved system for and method ofdetermining the position of one or more radio frequency identification(RFID) transponders with respect to one or more sensors is disclosed. Ina first embodiment, the system comprises a plurality of stationarysensor arrays located within certain physical areas. Each sensor arraycomprises a plurality of antenna coils arranged in unique physicalorientations with respect to each other and capable of transmittingradio frequency signals of differing phase. The RFID transponderincludes a magnetic loop antenna which receives the plurality of signalsgenerated by the antenna coils, and compares the phase of at least twoof the received signals in order to determine the relative position ofthe transponder(s) with respect to the sensors.

In a second aspect of the invention, an improved system for and methodof maintaining constant communication between one or more RFIDtransponders and their associated sensor(s) using a multi-messageprotocol is disclosed. In one embodiment, the system comprises sensorarrays having a plurality of antenna coils in a predetermined physicalrelationship which emit two direction finding mode (DFM) signals insuccession. For the emission of the first DFM signal, each of theplurality of antenna coils is energized so as to emit a signal in itsgiven orientation. For the emission of the second DFM signal, one of theplurality of coils is turned off such that no radio frequency signal isemitted from that coil. The spatial relationship of the transponder andindividual antenna coils precludes all of the signals from each sensorarray from being rejected by the transponder during the emission of boththe first and second DFM signals. In this fashion, the transponder coilcan be kept in constant communication with the sensor, regardless of itsorientation with respect to the sensors. This feature effectivelyeliminates the communication problems associated with the typical“figure-8” pattern associated with the transponder's antenna coil.

In a third aspect of the invention, an improved method of determiningthe location of the transponder with respect to two or more sensorarrays through elimination of sensor rejection is disclosed. In oneembodiment, the method comprises positioning the transponder with aninternal antenna coil within the field generated by the coils of theindividual sensor arrays, transmitting a first signal from each of theantenna coils of the two or more arrays, transmitting a second signalfrom a subset of the antenna coils of the same arrays (i.e., with one ormore coils turned off), and determining the position of the transponderrelative to the two or more sensor arrays based on the intensity of thefirst and second signals received by the antenna coil of thetransponder.

In a fourth aspect of the invention, a system for and method oftransmitting data between a sensor having a transmit coil and a RFIDtransponder having a receiving coil is disclosed. In one embodiment, thesystem comprises a hand-held sensor probe or wand which emits a highlyintense and localized wake-up field at a predetermined frequency. Thisfield is sensed by the receiving coil of the transponder, and itsphysical parameters (such as intensity and/or frequency) compared topredetermined values present within the transponder. If the sensedparameters of the wake-up field meet certain predetermined criteria, thetransponder generates an internal wake-up signal, and beginscommunicating with the sensor. This system and method are particularlyuseful when using transponders having an internal motion detector,thereby allowing communication with the dormant (e.g., motionless)transponder without the need to physically move the transponder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of the magnetic field sensitivity pattern of atypical prior art loop antenna having a circular shape, based on 10 dbdownpoints of field strength.

FIG. 2 is diagrammatic representation of the three carrier wave phasevectors associated with the present invention.

FIG. 3 is a plot illustrating the positional angle measured by thetransponder of the present invention as a function of the intensity ofthe signals received by the transponder from two three phase sensorarrays.

FIG. 4 is a plan view of an exemplary arrangement of the three phasesensor array of the present invention, placed with respect to a door.

FIG. 5 is a logical block diagram showing a portion of thecommunications between an RFID sensor and transponder of the presentinvention.

FIG. 6 is a block diagram of one embodiment of a stand-alone PC-basedasset management system with network link according to the presentinvention.

FIG. 7 is a block diagram of one embodiment of a security system withnetwork link according to the present invention.

FIG. 8 is a block diagram of one embodiment of the sensor hardware andarrays shown in FIGS. 4, 5, and 6.

FIG. 9 is a block diagram of one embodiment of the sensors used in thesensor arrays of FIG. 8.

FIG. 10 is a schematic diagram of the three phase transmitter andantenna array portions of the sensor shown in FIG. 9.

FIG. 11 is a physical assembly diagram showing one embodiment of theantenna coil configuration of the sensor arrays shown in FIGS. 8 and 9.

FIGS. 12A and 12B are diagrams of one embodiment of the end coils (leftend and right end, respectively) for the Z-direction of the antenna coilconfiguration shown in FIG. 11.

FIG. 13A is a diagram of one embodiment of the X-direction coil of theantenna coil configuration shown in FIG. 11.

FIG. 13B is a diagram of one embodiment of the Y-direction coil of theantenna coil configuration shown in FIG. 11.

FIG. 14 is a block diagram of one embodiment of the hardwareconfiguration for the RFID transponder shown in FIG. 5.

FIG. 15 is a plan view of the sensor array portion of the assetmanagement and security system of the present invention in an exemplaryinstallation, illustrating characteristic patterns of the transponderwake up range and sensor read range.

FIG. 16 is a logical state chart for the internal states of oneembodiment of the transponder of FIG. 14.

FIG. 17 is a schematic diagram of a portion of one embodiment of thetransponder wake up circuit of the transponder of FIG. 14.

FIG. 18 is a logical flow chart illustrating one embodiment of the twomessage direction finding mode (DFM) process performed by the assetmanagement system of the present invention.

FIG. 19 is a logical flow chart of one embodiment of the positionlocation technique utilized by the asset management system of thepresent invention.

FIG. 20 is a block diagram of one embodiment of the portable wake upsensor probe and system according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference is now made to the drawings wherein like numerals refer tolike parts throughout.

As used herein, the terms “tag” and “transponder” are usedinterchangeably, and are meant to include any mobile or semi-mobileradio-frequency device capable of transmitting, receiving, or bothtransmitting and receiving radio frequency energy in one or morefrequency bands. Similarly, the terms “sensor”, “transceiver”, and“reader” are used interchangeably, and are meant to include any fixed orsemi-mobile radio frequency devices capable of receiving, transmitting,or both receiving and transmitting radio frequency energy in one or morefrequency bands.

Fundamental Operating Principles

The fundamental operating principles of the present invention are nowdescribed with reference to FIGS. 2 through 4. The present inventionbroadly comprises a system by which an RFID transponder may communicatewith an array of RF sensors, the signals emitted by the sensors aidingthe transponder in locating its position relative to the sensors. Asshown in FIG. 2, the present invention utilizes sensors (antenna coils)which emit three carrier waves each separated by a phase angle θ, thephase angle in the present embodiment being equal to 120 degrees. Inanother embodiment, other phase angles are envisioned. The three carrierwaves are described by vectors where the magnitude of the magnetic fieldintensity H is described by:

H=sin(ωt +θ)  Equation 1

Where:

θ=0, 120, and 240 degrees (0π, 2/3π, and 4/3π radians), respectively,and

ω=angular frequency (radians/sec).

Each of the three phase-shifted carrier signals is transmitted by acorresponding loop antenna coil (see discussion of FIGS. 11-13 below),each antenna coil having its own “figure-8” pattern. Each of the threeantenna coils is arranged on one of three axes of a Cartesian (i.e., X,Y, Z) coordinate system. Hence, the magnetic field intensity around thecoils varies both as a function of position and time. Accordingly, theforegoing antenna coil and three phase carrier arrangement can be usedto both transmit data to the transponder, and also provide informationto the transponder regarding its position relative to the coils.

As shown in FIG. 3, the system of the present invention utilizes therelative strength or intensity of the electromagnetic three phasesignals radiated by two (or more) sensor arrays, herein designated as“A” and “B”, in order to provide position information to thetransponder. Specifically, the positional angle (relative to the antennacoils) is shown to be a function of the relative signals strengthspresent from the A and B arrays. The ratio of the signal strengths isdefined as:

Ratio_(AB=)20*log₁₀(I _(A) /I _(B))  Equation 2

Where:

I_(A)=Intensity of received signal from “A” sensor array, and

I_(B)=Intensity of received signal from “B” sensor array.

Hence, by knowing the intensity of the signals received from two antennaarrays, the tag may compute its positional angle relative to the knownposition of the arrays.

Referring now to FIG. 4, one exemplary embodiment of the three phasesensor array of the present invention is described in order to furtherillustrate the foregoing principles. As shown in FIG. 4, the antennaarrays of the exemplary embodiment are arranged in pairs 404, 406, 408,where one antenna coil assembly 402 of each pair is defined as being inone location relative to a door 410 (i.e., “inside” a building or anarea in the present illustration, designated by the letter “A) and theother coil assembly of each pair is defined as being in a secondlocation relative to the door 410 (i.e., “outside” of the building or anarea, designated by the letter “B”). The coil spatial and phaseorientation is shown for an “N” pair system. Note that each of the coilassemblies contains three antenna phase coils having a differingelectrical phase as previously described. The physical orientation ofthe coils may be as defined in FIG. 1. It will be appreciated that theillustrated system of sensor arrays may be set up over an infinitelylong barrier if desired, or configured in other arrangements.

RFID Asset Management System

Referring now to FIG. 5, the operation of one embodiment of the RFIDasset management system of the present invention is described in detail.As shown in FIG. 5, the system is comprised generally of a radiofrequency reader or sensor system 510 and a corresponding transponder520 which are each in communication with one another. Initially, thetransponder 520 is dormant or in an inactive state when outside thefield generated by the sensor(s) 510. This dormant state reduces thepower consumption of the transponder, since its internal processor 1406(FIG. 14) and other components are inactive. The sensor system 510generates an electromagnetic “wake up” field 130 using an alternatingcurrent (AC) signal as is well known in the art; see the discussion ofFIG. 15 below for more details regarding the sensor field pattern invarious applications. This wake up field is generated by the sensorsystem 510 in order to toggle the transponder 520 into the active statewhen the transponder 520 enters the wake up field (i.e., when the fielddetected by the internal magnetic loop antenna 1402 of the transponderas illustrated in FIG. 14 is of sufficient intensity), and thetransponder is in motion. In one embodiment, the sensor system 510communicates with the transponder 520 using an 8.13 kHz phase modulatedsignal, although it will be appreciated that other frequencies and/ormodulation schemes may be used. It will further be recognized thatbroadband (i.e., “spread spectrum”) radio frequency techniques such asDSSS or FHSS may also be used, although the descriptions of theexemplary embodiments contained herein are in terms of a non-spreadspectrum system. The transponder 520 then enters an “active” state andcontinues the communications protocol as generally illustrated in FIG.5. Specifically, the transponder 520 next initiates an anti-clashroutine 532 with the sensor system 510, after which the sensoracknowledges the transponder ID (including a cyclic redundancy code orCRC) and issues a “rename” command 534 to the transponder 520 in orderto assign it to one of a plurality of time slots for the directionfinding mode (DFM), explained in greater detail below with respect toFIG. 18. At this point, the sensor 510 requests information 536 as tothe location of the transponder in the sensor field(s) from thetransponder 520. The transponder 520 then responds with the requestedlocation information 538, which is derived using one of a variety oftechniques such as the relative signal strength method described withrespect to FIG. 19 herein. Hence, the transponder 520 is used in thepresent invention to supply, inter alia, information regarding itsposition to the sensor(s) 510.

Referring now to FIG. 6, one embodiment of a stand-alone PC-based assetmanagement system with network link incorporating the three phaseprinciples of the present invention is described. As shown in FIG. 6,the asset management system 650 generally comprises a personal computer(PC) based sensor processing system 600 coupled to a host computersystem 620 via one or more data links 612, 614. In the presentembodiment, the data links 612, 614 comprise network interfaces such asthose commonly associated with a local area network (LAN) or wide areanetwork (WAN) of the type well known in the art, although it will beappreciated that many other different types of data links such as SONETor wireless interfaces such as those compliant with IEEE Standard 802.11may be used. Quite literally any interface capable of transferringinformation from one component to another can be substituted. The sensorprocessing system 600 comprises one or more sensor processing subsystems601, 630 which each include a PC 606, 636 coupled to a plurality ofindividual daughter boards 602, 608, 610 and 632, 638, 640,respectively, each board having one or more radio frequency sensor coils(not shown) disposed thereon. In the illustrated embodiment, thedaughter boards are connected to their respective PCs 606, 636 viafrequency, coherent phase (F/2F) format data links, such as links 604,634, of the type well known in the magnetic recording arts, although itwill be appreciated that other types of data links may be used.

Referring now to FIG. 7, one embodiment of a security system withnetwork link incorporating the present invention is described. As shownin FIG. 7, the system 700 comprises a microcontroller 702 having anasset management sensor antenna array 703, a plurality of accesssensors/card readers 704, 706, and a door strike 708 coupled thereto.The microcontroller 702 includes an asset management sensor daughterboard (not shown), the latter which permits the microcontroller tointerface with the asset management sensor array 703. The asset managersensor array 703 includes one or more sensor arrays 402 of the typepreviously described for communication with one or more transponders520. The microcontroller 702 is further coupled to a host computer 620such as a personal computer or minicomputer via an interface processor(not shown) and a data link 710 of the type well know in the dataprocessing arts. The microcontroller 702 in the illustrated embodimentis a Model No. Micro-5 microcontroller manufactured by the Casi-RuscoCorporation, although other devices may be substituted. Themicrocontroller 702 receives an asset management database from the hostcomputer 620 during operation, and all assets entering the sensorsystem's range are stored and forwarded to the host computer 620.Conventional card readers 704, 706 are coupled to the microcontroller702 to allow personal access and asset management using the same system.

Referring to FIG. 8, one embodiment of a sensor hardware configurationof the antenna coil assemblies 402 and sensor system 510 of FIGS. 4 and5, respectively, is now described. In the illustrated embodiment, thesensor system 510 is configured such that the antenna coil arrays 402are disposed in pairs 404, 408 in relation to a door, an opening, avirtual barrier, or other location (shown generically as a door 410 inFIG. 4). As in FIG. 4, a set of sensor arrays 402 are partitioned onopposite sides, shown as side “A” and side “B” of the door 410. Thearrays 402 may be factory set (for example, set for field strength,location, orientation, etc.), or set by an installer or the end user.The sensor arrays 402 are each connected to a sensor bus 832. In theillustrated embodiment, the sensor bus is configured according to theRS-485 format, although other arrangements may be used. The sensor bus832 connects to a sensor interface 840, which is further connected tothe host computer 620 (FIG. 6). The sensor interface 840 includes anindustry standard 8051 type microprocessor, and random access memory(RAM) and read-only memory (ROM) circuits (not shown) to track thelocations of the transponders and to process communications with thesensor arrays and with the host computer. It will be recognized,however, that other components and configurations may be used in placeof the microprocessor/ROM/RAM combination described herein.

The sensor arrays 402 of the present embodiment may be configured asbar-shaped units located on the top or side of a door, or above a dropceiling, although other configurations are possible. The antenna coilsof the sensor arrays 402 emit an interrogating or wake up field to thetransponders, and deliver the transponder identification data receivedfrom the transponder(s) 520 to the host computer 620. The location ofthe transponder with respect to the sensor arrays 402 (i.e., “A” side or“B” side) is also provided to the host computer 620, using the methoddescribed with respect to FIG. 16 below. Additionally, the sensor system510 establishes whether the transponder 520 has passed through thevirtual barrier. This feature aids in entry control to restricted areas,as well as associating an asset with a person. The host computer 620 isalso warned of any condition that may compromise the sensor system 510or transponder 520 capabilities (e.g., jamming) by decoding the statusbyte sent by the tag during the anti-clash process. If the tagdetermines that there is interference in the band where the tag islistening for communication from the sensor, the tag emits the jammingalert in an un-synchronized manner.

Referring to FIG. 9, one embodiment of a sensor array, such as sensorarray 402 shown in FIGS. 4 and 8, will be described in detail. In theillustrated embodiment, the sensor arrays 402 receive an ultra highfrequency (UHF) response from a transponder 520 (FIG. 5) at a set of UHFantennas (“1 of N” antenna 902 through “N of N” antenna 904). Each ofthese antennas 902, 904 sends the received signal to a set ofcorresponding UHF receivers (“1 of N” receiver 912 through “N of N”receiver 914). Each of the UHF receivers 912, 914 processes the receivedUHF signal and sends its output to a message decoder and encoder (MDE)circuit 920. The MDE 920 includes an the input and output (I/O)interface 921 for the sensor array with the sensor interface 840 shownin FIG. 8. Master timing, transmit signals, and receive signals are allcommunicated on the I/O interface of the MDE 920.

The UHF receivers (902-904) operate on a multichannel frequency and areconfigured so as to be capable of continuous reception. A UHF antenna1414 (FIG. 14) of the transponder 520 is configured to transmit data tothe sensor system 510 using a 434 MHz nominal amplitude modulated (AM)carrier. The data bits are biphase coded using half bits of 330microseconds in the illustrated embodiment. Biphase coding is well knownin the art, and accordingly will not be discussed further herein. Signalstrength information from the UHF receivers informs the message encoder920 of incoming RF energy from the transponders 520, such as a burst ofdata. The UHF receivers send the signal levels and any received data tothe message encoder 920 for communication with the host computer 620 viathe sensor interface 840.

As previously noted, the sensor array 402 also generates a “wake up”field 530 directed to the transponder(s) 520. The door sensor interface840 sends master timing and other control signals to the MDE 920. TheMDE 920 decodes the message from the sensor interface 840 and sends thesignals to the master timing decoder 922. The output of the mastertiming decoder 922 is connected to a three phase modulator/frequencygenerator 930. The output of the three phase modulator/frequencygenerator 930 is input to a three phase amplifier 932 which thensupplies the three phase antenna array coils 940. The three phaseantenna array coils 940 generate the wake up field 530 which istransmitted to the transponders 520. The three phase amplifier 932 andthe three phase antenna array coils 940 comprise a three phasetransmitter circuit and are described in greater detail in conjunctionwith FIG. 10.

FIG. 10 illustrates one exemplary embodiment of the three phaseamplifier 932 and antenna array coils 940 of the door sensor shown inFIG. 9. As shown in FIG. 10, the amplifier 932 and antenna array 940comprise three discrete circuit phases 1000, 1030, 1060 which areelectrically coupled to corresponding antenna elements 1002, 1032, 1062.The three phases 1000, 1030, 1060 each take a respective input signal1004, 1034, 1064 from the three phase modulator/frequency generator 930(FIG. 9), amplify the signal, and output the amplified alternatingcurrent signal to the respective antenna element 1002, 1032, 1062. Asdescribed with reference to FIG. 2 above, the signals for each phase1000, 1030, 1060 of the illustrated embodiment are shifted by 120degrees of electrical phase as is well known in the electrical arts.Accordingly, the electromagnetic radiation emitted from the respectiveantenna elements 1002, 1032, 1062 is correspondingly shifted in phasesuch that magnetic and electric field intensity at a given point inspace attributable to each antenna element is also shifted in phase.While the illustrated circuit phases 1000, 1030, 1060 employ, interalia, operational amplifiers 1006, 1036, 1066 to perform theamplification function, it will be recognized that a variety of othercomponents and arrangements well know in the art may be used to amplifythe input signals 1004, 1034, 1064.

Referring now to FIG. 11, one embodiment of the physical configurationof the antenna transmit/receive coils 940 within the sensor arrays 402of the present invention is described. Each sensor array 402 comprisesgenerally three antenna elements 1002, 1032, 1062 which are electricallycoupled to the output of the circuit phases 1000, 1030, 1060 as shown inFIG. 10. The antenna elements 1002, 1032, 1062 are physically comprisedof loops of conductive material which are arranged so as to generatemagnetic flux in three orthogonal axes (e.g., the X, Y, and Z axes in aCartesian system) when alternating electrical current is passed therethrough. Specifically, the sensor array illustrated in FIG. 11 iscomprised of a substantially rectangular “X” antenna coil 1002, asubstantially rectangular “Y” antenna coil 1032, and a pair ofsemi-circular “Z” end coils 1061, 1063 electrically arranged in aHelmholz circuit of the type well known in the antenna arts. TheHelmholz arrangement of the end coils 1061, 1063 provides theZ-direction with an antenna aperture similar to that of one of thelarger X or Y coils 1002, 1032, thereby providing a more uniform fieldintensity in each of the three axes.

In a second embodiment, the antenna coils 1002, 1032, 1062 of the sensorarray 402 are comprised of a ferrite loaded material wound with aplurality of turns of thin (i.e., 20 ga.) copper wire. Other antennacoil configurations may be used as well.

The four antenna coil elements 1002, 1032, 1061, 1063 are physicallyconnected to their respective circuit phases (and to each other in thecase of the end coils 1061, 1063) via plug-in connectors 1120, 1122,1124, 1126 mounted on a printed circuit board 1100 located within thesensor array housing cover 1106. The cover 1106 forms a portion of theexterior housing of the sensor array 402, which is also comprised of twoend covers 1102, 1103 and a base plate 1104. The PCB 1100 is affixed tothe base plate 1104, as are the end covers 1102, 1103 and the housingcover 1106. The end covers 1102, 1103, base plate 1104, PCB 1100, andhousing cover 1106 cooperate to maintain the antenna coils 1002, 1032,1061, 1063 in the desired physical alignment (i.e., the X and Y coilsorthogonal to each other as well as the two end coils) when the sensorarray 402 is assembled. The housing cover, end covers, and base plateare constructed of plastic, plastic, and metal, respectively. Thesematerials are chosen for their magnetic permeability with respect to theRF energy generated or received by the antenna coils. It will berecognized by those of ordinary skill in the mechanical arts, however,that a wide variety of housings or structures useful for maintaining theantenna coils in the desired positions, as well as materials ofconstruction thereof, may be substituted for the arrangement of FIG. 11.

FIGS. 12A through 13B further illustrate the sensor antenna coilarrangement and direction of electrical current flow of each coil in thesensor array of FIG. 11. FIGS. 12A and 12B are diagrams of the end coils1061, 1063, respectively, for the Z axis. FIGS. 13A and 13B illustratethe X-direction coil 1002 and Y-direction coil 1032 of the sensor array,respectively.

Referring now to FIG. 14, one embodiment of the internal hardwareconfiguration for the asset transponder 520 of FIG. 5 is described. Aspreviously described, a low frequency tuned loop antenna 1402 receivestransmissions from the sensor system 510 (FIG. 5). In one embodiment,the low frequency used is approximately 8 kHz, although frequencies inthe range of 0 to 1 GHz may also be utilized. The transmissions receivedby the transponder include, inter alia, the wake up field 530 generatedby the sensor system 510 (FIG. 5) as well as other data such as the“rename” command 534 and request for location information (DFM) 536. Theoutput of the loop antenna 1402 is fed to a wake up detector andreceiver circuit 1404. The wake up detector portion of the circuit 1404detects a wake up field received by the transponder loop antenna 1402 bymeasuring the intensity of the received signal, and the receiver portionof the circuit 1404 receives and processes the low frequency datasignals received by the loop 1402. The wake up detector and receivercircuit 1404 is further described in conjunction with FIG. 17 below.

As shown in FIG. 14, the output of the circuit 1404 feeds acommunications processor 1406 disposed within the transponder 520. Theprocessor 1406 is responsible for the processing of, inter alia,portions of the anti-clash, CRC, and ID code functionality describedbelow. The processor of the present embodiment is a Model No. 161V588-bit processor manufactured by Microchip, although other types ofprocessor may be used. The system can also be encoded into a statemachine and developed as an application specific integrated circuit(ASIC). A tamper detector 1410 and/or motion sensor 1411 are alsointerconnected with the processor 1406. The tamper detector 1410 helpsdetermine if attempts are being made to tamper with the transponder 520,while the motion sensor 1411 assists the processor in determining if thetransponder 520 is in motion. The tamper detector 1410 of theillustrated embodiment comprises a switch-type arrangement using anormally closed switch of the type well known in the electrical arts,although other types of detectors may be used. The motion sensor 1411is, in the present embodiment, a bi-morphous accelerometer of the typewell known in the art, although it will again be recognized that othertypes may be substituted.

The processor 1406 of the transponder 520 further connects to atransmitter circuit 1412 which in turn supplies a tuned ultra-highfrequency (UHF) loop antenna 1414. The UHF loop 1414 is used for sendingmessages, such as the ID code, to the sensor system 510. The UHF loop ofthe illustrated embodiment operates at a frequency of 434 MHz, althoughother frequencies may be used. In one embodiment, the UHF loop does notreceive any signal from the sensors.

In the present embodiment, the transponder 520 remains dormant untileither: (i) the motion sensor 1411 is activated by motion of thetransponder 520 and a low frequency, e.g., 8 kHz, RF signal is detected;(ii) the tamper detector 1410 is activated (such as by someone trying toopen the transponder 520 or remove it from the asset to which it isattached); or (iii) a high-intensity localized wake up field is receivedas described with respect to FIG. 20 herein.

When the 8 kHz wake up field is detected, the transponder emits its IDcode at the respond command from the sensor system 510. The 8 kHz fieldis modulated, thereby permitting data transfer from the sensor system510 to the tag 520 via the signal. When the sensor system detects the IDcode of the transponder (which is transmitted back to the sensor systemby the transponder loop antenna), the sensor system 510 records thetransponder ID and instructs the transponder 520 to stop transmitting.Subsequently, the sensor system 510 transmits a direction findingcommand to check for the position of the transponder 520. When thetransponder 520 is in the control of the sensor system 510, the rate ofinterrogations by the sensor system varies according to loading.Specifically, as more transponders enter the RF field, the number oftimes per second that a given transponder is located is reduced. Thisreduces the speed of the radio location system as a whole, butadvantageously does not impede the ability of the system to detect orcommunicate with additional transponders 520.

FIG. 15 is a plan view of a barrier (e.g., door 410) illustrating thecharacteristic sensitivity patterns 1500 of one exemplary installationof a sensor array 402 and sensor system 510 according to the presentinvention. Shown in FIG. 15 are (i) the maximum sensor read range 1502(dependent on radiated frequency and power) for a three phase emitter;(ii) the maximum wake up range 1510 of the transponder 520 when thetransponder antenna is parallel to the X axis (FIG. 1) of one emittercoil of the antenna array for the illustrated embodiment; and (iii) themaximum wake up range 1512 of the transponder 520 when the transponderantenna is perpendicular to the X axis (FIG. 1) of one emitter coil ofantenna array 402 and only one coil is energized. The direction ofmovement of a hypothetical tagged asset moving through the door 410 isalso shown by arrow 1508. Vector 1504 illustrates the maximum usefulrange achievable by the system 510, based on the overlap of the maximumwake up range and sensor array range. In the present embodiment, thisrange is approximately 6 m, although other distances may be useddepending on the needs of the user and the specific application.Furthermore, it is contemplated that other sensitivity patterns may becreated depending on such considerations.

FIG. 16 illustrates the various states of the transponder 520 of anexemplary embodiment of the present invention during operation of thesystem. As illustrated in FIG. 16, the transponder(s) 520 generallyoperates in one of seven states, namely: (i) the “stand by” state 1602;(ii) the “reset” state 1606; (iii) the “respond” state 1610; (iv) the“active” state 1614; (v) the “inhibit” state 1618; (vi) the “renamed”state 1624; and (vii) the direction finding mode (DFM) tracking pulsestate 1630. These states and their inter-relationships are describedbelow. It is noted that while discussed in terms of logical states, theoperation of the transponder 520 as described herein is achieved usingone or computer algorithms running on the microprocessor 1406 of thetransponder, as well as the host computer processor or otherintelligence associated with the sensor system 510. Such computeralgorithms are ideally stored in the volatile and non-volatile memorydevices (i.e., ROM, RAM, and magnetic storage media) incorporated withinthe transponder and sensor system, although other arrangements andstorage schemes may be used.

In the stand by state 1602, electrical current consumption is reduced tominimum levels, and the transponder 520 waits for wake up signal. Duringthis state, the microprocessor 1406 in the transponder is inactive. Thetransponder enters the reset state 1606 when the wake up signal 532 isreceived.

In the reset state 1606, the transponder is waiting for a transmissioncommand to be issued by the sensor system 510. The wake up receivercircuit (FIG. 17) has seen a continuous signal for the minimumpredetermined wake up time and activates the microprocessor 1406. Thetransponder waits for a “respond”, “retry” or “reset”command before itsends its ID information to the sensor system 510 and enters the respondstate 1610.

In the respond state 1610, the transponder sends its ID to the system510. First, in one embodiment, a preamble (tag activity pulse, or TAP)is sent. The TAP informs the sensor system 510 that at least onetransponder 520 is responding. A random number generator within thetransponder processor 1406 selects one of eight time slots fortransmission of the ID. The transmission of the actual ID starts at thisrandomly selected time slot. While waiting for this time slot, thetransponder observes the signals emitted by the sensor system 510; assoon as a “channel free” signal disappears, the transponder stops towait for transmission back to the sensor system and enters inhibit state1618.

The ID message sent from the transponder 520 (i) informs the sensorsystem 510 the ID code of the tag which responded; (ii)and the status ofthe alarm bits in the tag.

In the inhibit state 1618, the transponder 520 waits for a “retry”command that offers the next chance to transmit, since the free 434 MHzchannel was acquired by another transponder. The retry command returnsthe transponder to the respond state 1610 where the tag waits foranother respond command.

A transponder that has sent its ID on the available 434 MHz channelwithout being inhibited enters the active state 1614. All commandsissued by the sensor system 510 that are appropriate and should only beexecuted by a single, isolated transponder, are accepted in the activestate 1614 only. In the illustrated embodiment, the only such command isthe “rename” command (described below), although the use of other suchcommands is contemplated herein. While in active state 1614, thetransponder 520 also receives an acknowledgement to its ID transmission.This acknowledgement contains, inter alia, a cyclic redundancy code(CRC) of the type well known in the signal processing arts, as well asthe number of a time slot it has been allocated to use in directionfinding mode (DFM) by the sensor system 510. The CRC is checked, and ifvalid, the transponder is commanded to enter the renamed state 1624. Ifthe CRC is not valid, the transponder enters the inhibit state 1618 andawaits a “retry” command.

The Rename command send a message to only the recently responding tag.The tag is identified by using CRC code as a unique identifier. Thecommand instructs the tag to use a predetermined time slot for allsuccessive DFM commands.

In the renamed state 1624, the transponder 520 waits for the DFMinitiation command from the sensor system 510 or, alternatively, a“reset” command. If the DFM initiation command is received, thetransponder enters the DFM tracking pulse state 1630. If the “reset”command is received, the transponder is returned to the respond state1610 as shown in FIG. 16.

In the DFM tracking pulse state 1630, the transponder first waits forthe next DFM tracking command header, the header being used tosynchronize all the tags being in DFM to the same time base. The DFMtracking command of the present embodiment comprises generally a DFMheader field, tracking opcode (four bits), and a DFM groupidentification code (also four bits). The header synchronizes thetransponder with the sensor system's bit clock (not shown), and providesa secure indication to the transponder that a new message is commencing.The opcode provides the transponder with instructions for the directionfinding mode of operation. The DFM group is used to identify thetransponder in the event that a large number of transponders are in usesimultaneously.

When this next DFM command header is detected, the transponder waits forits DFM time slot (assigned via the “rename” command), and subsequentlysends the DFM directional information to the sensor system 510. Thetransponder then returns to the renamed state 1624 and awaits furthercommands form the sensor system 510.

It is noted that the embodiment of the invention illustrated in FIG. 16also includes an “emergency call” state 1646. In this state, thetransponder 520 issues an emergency message to the sensor system 510using the 434 MHz channel to alert the sensor system 510 of tampering orother predefined external digital input. The external input causes aninterrupt into the tag which starts a predetermined message to beemitted at 434 MHz. This message follows the protocol used by all tagsduring the respond command.

Referring now to FIG. 17, a schematic of one exemplary embodiment of thewake up circuitry 1404 of the transponder 1400 (FIG. 14) will bedescribed. In one embodiment, the transponder 520 receives a signal atthe receive antenna 1402 which is propagated to an 8 kHz input section1710. A pair of diodes D1 1703 and D2 1705, acting as a symmetricaldiode limiter, are added to stop overload of transistor Q5 1707 of the 8kHz section 1710 due to the stronger input level of the wake up signalfrom a Portable Transponder Sensor. The anode of D2 1705 connects topoint 1702 and the cathode connects to ground, while the cathode of D11703 connects to point 1702 and the anode connects to ground. In oneembodiment of the transponder, the voltage induced by the wake up signalat point 1702 is about one volt. The anode of a diode D3 1704 connectsto point 1702 and is used to generate a transponder wake up signal atpoint 1706. A capacitor C10 1711 connects between the cathode of D3 andground 1715. In one embodiment, the voltage level of the transponderwake up signal at point 1706 is about 0.3 volts. A resistor R32 1718,having a value of 10,000 ohms in one embodiment, which is connectedbetween the receive antenna 1402 and point 1702, is optional.

Elimination of Sensor Rejection by the Transponder

As previously described, the loop antenna of the transponder 520 is madefrom a magnetic coil based primarily on space considerations. The fieldpattern of the transponder coil antenna has a classical “donut” shapewell known in the art. This donut pattern exhibits a three dimensionalvector in opposite directions, whereby the transponder antenna canreject the signal from a sensor if the transponder is held in the properorientation (i.e., such that the sensor field is aligned along thelongitudinal axis of the donut). In the situation where the sensorsystem 510 comprises a “near” sensor and a “far” sensor (such as the “A”and “B” side sensor arrays shown in FIG. 4 ) when the transponder 520 islocated on one side of the door 410 or the other, if the transponder isoriented such that the signal from the near sensor array 402 fallswithin the solid angle of rejection of the transponder loop antenna, thesignal from that sensor could be attenuated by up to 30 dB. Since thesensor system determines position based at least in part on the relativestrength of the signals received from the sensor arrays 402, the sensorsystem 510 may erroneously interpret the location information toindicate that the transponder is closer to the “far” sensor (i.e., thesensor on the opposite side of the door 410) rather than the nearsensor.

To eliminate this type of error, the present invention createsconditions at the antenna of the transponder 520 such that the signalfrom the near sensor array cannot be rejected. Two techniques areutilized to achieve this result: (i) increasing the number of signalsbeing coupled into the transponder; and (ii) turning off one of theorthogonal phases of the sensor array 402 in order to eliminate the“cancellation” effect. Each of these techniques is described in moredetail below.

Increasing the number of signals being coupled to the transponderantenna is achieved by generating more than one signal at the sensorarray 402, as previously described with respect to FIGS. 2-4. Eachsignal generated by the sensor array must have a different polarization.Since there are only three spatial dimensions, a signal polarized toeach of these three dimensions (i.e., X, Y and Z) will account for allspatial orientations. As a result of more signals (each of which are ondifferent polarizations in the environment of the transponder), thesolid angle where the transponder 520 could possibly reject any onesensor array phase is dramatically reduced. However, as will berecognized by those of skill in the art, the coupling efficiency for anyof these signals to the transponder antenna depends also on theorientation of the transponder antenna. Accordingly, additional methodsare necessary to ensure that the transponder antenna can not reject thesignal from all of the sensor array phases regardless of thetransponder's orientation.

The foregoing limitation is addressed by the second technique employedby the present invention; namely, electrically “turning off” one of thethree sensor array phases. Specifically, it will be recognized that ifthe transponder 520 is held in a position in space where the threesignals from the nearest sensor array 402 are attenuated, then thesesignals are necessarily attenuated through a cancellation effect. Thecancellation effect is caused by two signals coupling into thetransponder coil, and a third signal also coupling in to the transpondercoil, but in the opposite direction. Specifically, referring to FIG. 1,the current in a coil is related to the phase of the magnetic fieldgenerated by the current in accordance to Lenz's law. This is also knownas the “right hand rule”. Substituting the current in the wire withrotating vectors, then the vector sum of the currents induced into theloop in this example is as follows: when all three signals are turnedon, then a current is induced into the coil from each of the threephases. The resultant vector is the vector sum of the three signals. Itis possible for the vectors to combine such that the resultant vector iszero. For this to occur, the induced current must be equal for all threesignals. If this is so, then when one signal is turned off, the vectorsum can never be zero.

To eliminate this cancellation effect, one of the three loops of therejected antenna array 402 is turned off using a conventional switchingcircuit (not shown) within the sensor system 510. Advantageously, thechoice of the phase to be turned off is not critical, since the absenceof field emitted by any one phase will effectively eliminate thecancellation effect at the transponder under any orientation of thelatter. Since the rejected array will no longer be rejected (i.e., itssignal will no longer be reduced in intensity by the up to 30 dBpreviously noted), it will be properly recognized by the transponderloop and correlated to a correct angular position by the processingalgorithms previously described according to the relationshipillustrated in Equation 2 and FIG. 3 herein.

Additionally, the present invention employs a unique messagetransmission protocol between the sensor system 510 and thetransponder(s) 520. Specifically, the sensor system 510 transmits twomessages from the antenna coils within its arrays 402. The first messageis transmitted from all three phases of each sensor array 402, while thesecond is transmitted from only two of the three phases of each array.In this fashion, both of the messages emitted by the sensor arrays 402can not be rejected by the transponder 520 regardless of its physicalorientation with respect to the arrays, since the previously described“cancellation effect” is eliminated by turning one of the phases off.Stated differently, one of the two DFM messages transmitted by eachsensor array 402 necessarily must be received by the transponder.Accordingly, each transponder 520 is kept in effectively constantcommunication with the sensor system 510. If the two DFM messages aretransmitted in temporal sequence, the transponders 520 are incommunication with the sensor system during at least one of themessages, thereby maintaining continuity (albeit having “dead” timecorresponding to the rejected message transmission interposed therein).Furthermore, the two messages of the present DFM protocol are identicalin content, so even if a transponder rejects one of the two messages, nodata or other message content is lost.

Referring to FIG. 18, the aforementioned two message DFM method is nowdescribed in greater detail in terms of a preferred algorithm 1800running on the transponder and sensor system processors. Beginning at astart state 1802, the algorithm 1800 moves to a transmit state 1804where each of the sensors in the system transmits a first DFM messagewith all three antenna array phases being on. For example, in a systemwith an “A side” sensor array and a “B side” sensor array such asillustrated in FIG. 4 herein, both sensor arrays of each sensor pairwould transmit the first DFM message. Proceeding next to a decisionstate 1806, the algorithm determines if the transponder rejects thesignal from one of the sensor arrays. If so, the algorithm continues toanother state 1808 and, in one embodiment, turns off one of the threephases at each sensor. In the illustrated embodiment, a phase is turnedoff by sending a command to the generator circuit 930 (FIG. 9), althoughother ways of turning off one of the phases are contemplated. Proceedingnext to a second transmit state 1810, both of the sensor arrays of eachpair transmit a second DFM message with one of the three phases turnedoff. The phase which is turned off can be any one of the three, aspreviously discussed. Moving to a decision state 1812, the algorithm1800 again determines if the transponder rejects the signal. If so, inone embodiment, there is a system error (based on the fact that both thethree-phase and two-phase DFM messages can not physically be rejected bythe transponder), and the algorithm 1800 advances to an error processingstate 1814 for analysis of the transponder(s) 520 rejecting the secondsignal. This state compares the DFM position information returned fromthe tag from the two messages. If the position information is different,the data is discarded and two more DFM messages are sent. However, if atransponder does not reject the second “two-phase” DFM signal, asdetermined at the decision state 1812, the algorithm 1800 processes thecontent of the second DFM message per function 1816.

In one embodiment, the DFM message processing function 1816 isimplemented as a truth table. The two responses by each transponder tothe two transmissions from the sensors are compared in the followingexemplary truth table:

First Response Second Response Conclusion A side A side transponder isin the A side A side B side inconclusive B side B side transponder is inthe B side B side A side inconclusive

After the message processing function 1816 completes, the algorithm 1800advances to a decision state 1818 and determines if the function 1816yielded valid position data. If so, the algorithm moves to state 1820 toreport the position of the transponder to the sensor system 510 via thetransponder-to-sensor system communication 538 illustrated in FIG. 5.The algorithm 1800 then enters an end state 1822. If, however, it isdetermined at decision state 1818 that the position data is not valid,the algorithm 1800 moves back to the first transmit state 1804 to beginthe process again so as to try to obtain valid position data.

It is noted that if it is determined in decision state 1806 that none ofthe signals are rejected by the transponder, the algorithm 1800 advancesto state 1824. At state 1824, the strongest signal of the signalsreceived at the transponder antenna is determined, and the algorithmselects the strongest signal as indicative of the sensor that thetransponder is closest to in location. Alternatively, the transpondermeasures the relative signal strengths of those signals received anddevelops an estimate of the relative angular position as previouslydescribed with respect to FIG. 3.

Direction Finding Method

FIG. 19 illustrates the direction finding method of the presentinvention in greater detail. This method 1900 determines the position ofthe tag or transponder in relation to the multiple system sensor arrays402 disposed around an access point such as a door 410 (FIG. 4).Beginning at a start state 1902, the method employs a first decisionstate 1604 to determine if the transponder 520 is in motion. Recall thatthe transponder contains a motion sensor 1411 which enables the “wakeup” of the transponder processor if the transponder is both in motion(over a given time interval) and within an RF field of a predeterminedintensity. If the transponder is not in motion or the field not present,the method 1900 is terminated until it is determined that foregoingconditions are met. When these conditions are satisfied, the transponderattempts to synchronize with the commands being issued by the sensorarrays 402 in state 1906. Eventually, each of the sensor arrays of thesensor system 510 issues a Direction Finding Mode (DFM) command at state1908. In one embodiment of the invention, after the DFM command isissued, each of the sensors emits a plurality (e.g., thirty-two) cyclesof an 8130 Hz signal, followed by a lesser number (e.g., nine) cycles ofdirectional information at state 1910. It will be recognized that thesensor may emit a signal at another frequency or frequencies if desired.During the nine cycles of directional information, the “A” sensor arrayof the sensor pair transmits the nine cycles with a 180 degree phaseshift, while the “B” sensor does not phase shift the 8130 Hz signal. Forthe purposes of this discussion, a phase shift is accomplished byshifting a given number of cycles of an RF signal with respect to acenter or “normal” frequency. Thus, the signals from the “A” and the “B”sensor arrays are phase shifted by 180 degrees from each other duringthe period of the nine cycles.

In state 1912 of the method 1900, the receiving transponder waits untilthe aforementioned nine cycle period and compares the received signalfrom sensor “A”with the received signal from sensor “B”. This comparisonyields an indication of which of the two sensor arrays 402 (i.e., eitherthe “A” array or the “B” array) is closer to the transponder. In oneembodiment, this comparison is performed by rationing the relativesignal strengths of the signals from the two sensors arrays aspreviously described with respect to FIG. 3. If the relative phasedifference is 80 degrees or less, the transponder believes it is closerto the “B” sensor field; that is, on the “B” side of the door 410 ofFIG. 4. However, if it is determined at the decision state 1914 that thephase difference is greater than 100 degrees, then the transponderbelieves it is on the “A” side of the door. The transponder reports itsposition to the sensors system 510 accordingly in state 1916 (for the“B” side of the door 410) or state 1918 (for the “A”side of the door410), and the process completes at an end state 1920.

It will be recognized that while phase shifts of 80 and 100 degrees areused as decision criteria in the present embodiment, other decisioncriteria and in fact other approaches may be used to determine to whichof the sensor arrays the transponder 520 is closer.

Dormant RFID Transponder Communication System

Referring now to FIG. 20, a system and method for communicating with adormant RFID transponder is now described. In the embodiment of FIG. 20,the dormant transponder communication system 2000 comprises a portabletransponder sensor (PTS) 2002 which is advantageously attached to alaptop or other portable computer 2004. It will be recognized that otherelectronic computing devices, such as palmtop organizers, calculators,or even non-portable devices may be substituted for the illustratedlaptop computer 2004 if desired. The system is used to associate thetagged asset to the transponder 520, and once associated, thetransponder 520 can be checked on a routine or maintenance schedulewithout having to activate the motion detector 1411 or other sensorwithin the transponder.

The system 2000 of the illustrated embodiment is capable ofbi-directional communication with one or more transponders 520, andallows the user to perform maintenance and inventory control functionsassociated therewith. The PTS 2002 includes a processor (not shown), anon-board phase shift keying (PSK) signal generator 2001, and a 434 MHzreceiver section 2003, for generating commands and decoding F/2F data,such as non-return-to-zero coded data, which is received from thetransponder(s) 520. The processor may alternatively or simultaneouslycomprise the host processor of the computer 2004. All of the transmitand receive antennas 2012, 2014 are mounted within the PTS 2002 forcompactness and ease of use. The PTS 2002 is capable of generating amagnetic field of great enough intensity to allow for the dormanttransponder 520 (such as a transponder having a motion sensor which isnot in motion) to wake up when the PTS 2002 is placed in relativeproximity to the transponder 520. In the illustrated embodiment, the PTS2002 generates a field with intensity on the order of one Gauss,although other field intensities may be used.

The acquisition portion 2017 of the PTS 520 receives commands, anddownloads data to the laptop computer 2004 via a standard RS-232 serialdata link of the type well know in the data processing arts, althoughothers (such as USB, wireless, or infrared/optical coupling, forexample) may be readily used. The laptop 2004 of the illustratedembodiment has a Windows®-based interface with appropriate screens andmenu structures that direct or allow the operator to perform variousdesired functions relating to the transponder 520 or PTS 2002.

The PTS 2002 of the embodiment of FIG. 20 comprises generally a handheld wand having an LCD screen 2019 (with back lighting) capable ofdisplaying information relating to operation of the system 2000 and thetransponder 2002. The PTS 2002 further comprises an input device 2010such as a series of keys or pushbuttons on its outer surface whichpermit the operator to accomplish a variety of data input andpreprogrammed functions. A variable menu structure is also optionallyused, whereby individual keys of the input device 2010 may be used toperform multiple functions. It will be recognized that while theillustrated embodiment uses a key/menu arrangement and LCD screen forinformation display and input, other configurations such as atouch-sensitive screen (with or without stylus), cathode ray tube, orthin film transformer (TFT) or plasma display may be used with equalsuccess.

The PTS 2002 of FIG. 20 is further capable of changing the modes orstates of the transponder 520 with which it is in communication, such asfrom “normal” to “transport” mode or vice versa. Normal mode is a statewhereby the tag behaves as previously described. Note that the tag mustbe moved so that the accelerometer will wake up the microprocessor.However, this state is wasteful of battery life if the tag has not yetbeen installed, or is in transport and is constantly being shaken.“Transport mode” is a state where the tag does not use the signal fromthe accelerometer and so the tag enjoys a longer battery shelf life.

In another aspect of the invention, the PTS 2002 is able to receivetamper alert messages from the transponder 520 in an unsynchronizedfashion; these alert messages are generated within the transponder bythe processor 1406 when the transponder is tampered with. Specifically,the tamper detector 1410 provides a signal to the processor 1406 whenthe detector is activated (such as by someone trying to remove thetransponder from the asset). These signals may be stored by thetransponder for later retrieval by the PTS 2002, or directly convertedto a radio frequency message emitted by the transponder and received bythe PTS when the transponder is tampered. Information received by thePTS 2002 further includes, inter alia, the ID of the tampered withtransponder.

While the above detailed description has shown, described, and pointedout fundamental novel features of the invention as applied to variousembodiments, it will be understood that various omissions,substitutions, permutations, and changes in the form and details of theapparatus and methods illustrated may be made by those skilled in theart without departing from the spirit of the invention. The foregoingdescription is not in any way meant to limit the scope of the invention;rather such scope is determined by the claims appended hereto.

What is claimed is:
 1. A method of determining the position of a radiofrequency identification (RFID) transponder with respect to a pluralityof transceivers, said transceivers having a plurality of individualantennae capable of transmitting radio frequency signals, the methodcomprising: transmitting a first signal having a first phase on at leastone of said individual antennae of each transceiver; transmitting asecond signal having a second phase on at least one of said individualantennae of each transceiver; receiving said first and second signals atsaid transponder; and comparing the phase of at least said first andsecond signals in order to determine the relative position of saidtransponder with respect to said transceivers.
 2. The method of claim 1,wherein the acts of transmitting a first signal and a second signalfurther each comprise transmitting signals of differing phase on each ofa plurality of individual antennae within each of said transceivers. 3.The method of claim 2, wherein said individual antennae of eachtransceiver are orthogonal to all other antennae within the sametransceiver.
 4. The method of claim 1, wherein the act of comparing thephase of said at least first and second signals comprises measuring therelative intensity of each of said first and second signals.
 5. Themethod of claim 4, wherein the act of comparing the phase furthercomprises calculating a ratio of the intensity of said first and secondsignals.
 6. The method of claim 5, wherein the act of comparing thephase further comprises correlating the ratio of the intensity of saidfirst and second signals to said relative position.
 7. The method ofclaim 2, wherein each of said transceivers comprises three individualantennae, and the signals transmitted by each of said individualantennae of each transceiver are separated by 120 degrees of electricalphase.
 8. The method of claim 1, wherein said first signal and saidsecond signal are transmitted continuously.
 9. The method of claim 8additionally comprising: determining when said transponder is in motion;and processing said first and second signals by said in motiontransponder so as to determine the relative position of said in motiontransponder with respect to said transceivers.
 10. The method of claim8, additionally comprising: determining when said transponder is beingtampered with; and processing said first and second signals by saidtampered transponder so as to determine the relative position of saidtampered transponder with respect to said transceivers.
 11. The methodof claim 1, wherein the relative position of said transponder withrespect to said transceivers is determined within said transponder. 12.A radio frequency positioning system, comprising: at least one radiofrequency identification transponder having at least one antennadisposed therein, said at least one antenna configured to receive radiofrequency signals; a first coil oriented in a first direction relativeto a given point; and a second coil oriented in a second directionrelative to said given point, said second coil being in differentphysical location than said first coil, said first and second coilsconfigured to transmit first and second radio frequency signals,respectively, wherein said first and second signals transmitted by saidfirst and second coils are different in phase, said phase differencebeing used by said transponder to determine a position of thetransponder relative to said first and second coils.
 13. The system ofclaim 12, further comprising: a third coil oriented orthogonal to saidfirst coil; a fourth coil oriented orthogonal to said first and thirdcoils, said first, third and fourth coils being physically co-located; afifth coil oriented orthogonal to said second coil; and a sixth coiloriented orthogonal to said second and fifth coils, said second, fifth,and sixth coils being physically co-located; wherein said third, fourth,fifth, and sixth coils are each configured to transmit radio frequencysignals.
 14. The system of claim 13, wherein said first, third, andfourth coils transmit signals which differ in electrical phase.
 15. Thesystem of claim 14, wherein said second, fifth, and sixth coils transmitsignals which differ in electrical phase.
 16. The system of claim 12,wherein said transponder measures the relative intensity of said firstand second signals in order to determine said position of thetransponder.
 17. The system of claim 13, wherein at least one of saidfirst, third and fourth coils are arranged in a Helmholz configuration.18. The system of claim 12, wherein said first and second radiofrequency signals are transmitted continuously.
 19. The system of claim18, wherein said transponder includes a motion sensor operable todetermine when said transponder is in motion, such that when it isdetermined that said transponder is in motion, said first and secondsignals are processed by said transponder so as to determine therelative position of said transponder with respect to said transceivers.20. The system of claim 18, wherein said transponder includes a tamperdetector operable to determine when said transponder is being tamperedwith, such that when it is determined that said transponder is beingtampered with, said first and second signals are processed by saidtransponder so as to determine the relative position of said transponderwith respect to said transceivers.
 21. A method of determining theposition of a radio frequency identification (RFID) transponder withrespect to a transceiver, said transponder having an antenna coilconfigured to receive a radio frequency signal, said transceiver havinga first sensor and a second sensor in different locations, said firstand second sensors further including at least one antenna coil, themethod comprising: positioning said transponder in a first spatialrelationship with respect to said first and second sensors; transmittinga first signal having a first phase from said at least one antenna coilof said first sensor; transmitting a second signal having a second phasefrom said at least one antenna coil of said second sensor; anddetermining the position of said transponder relative to said first andsecond sensors based on the phase relationship between said first andsecond signals received by said antenna coil of said transponder. 22.The method of claim 21, wherein the act of transmitting furthercomprises transmitting signals of differing phase on each of a pluralityof individual antenna coils within each of said sensors.
 23. The methodof claim 22, wherein said individual antenna coils of each sensor areorthogonal to all other antenna coils within the same sensor.
 24. Themethod of claim 21, wherein the act of determining the position of saidtransponder comprises measuring the relative intensity of each of saidfirst and second signals.
 25. The method of claim 24, wherein the act ofmeasuring the relative intensity further comprises calculating a ratioof the intensity of said first and second signals.
 26. The method ofclaim 22, wherein each of said sensors comprises three individualantenna coils, and wherein the signals transmitted by each of saidindividual antenna coils are separated by 120 degrees of electricalphase.
 27. The method of claim 21, wherein the determining of theposition of said transponder is performed by said transponder.
 28. Themethod of claim 21, wherein said first signal and said second signal aretransmitted continuously.
 29. The method of claim 28 additionallycomprising: determining when said transponder is in motion; andprocessing said first and second signals by said in motion transponderso as to determine said relative position of said in motion transponderwith respect to said transceivers.
 30. The method of claim 28additionally comprising: determining when said transponder is beingtampered with; and processing said first and second signals by saidtampered transponder so as to determine the relative position of saidtampered transponder with respect to said transceivers.
 31. A method ofdetermining a position of a radio frequency transponder regardless ofits orientation, the method comprising: providing at least two sensorseach having a plurality of transmit coils capable of transmitting radiofrequency energy within a predetermined frequency band; placing each ofsaid transmit coils of each of said at least two sensors in a differentorientation with respect to a given reference point; providing areceiver coil within said transponder; generating radio frequencyemissions from each of said transmit coils of each of said at least twosensors, said emissions being shifted in electrical phase from eachother; and measuring the phase of signals received by said receiver coilto determine the position of the transponder.
 32. The method of claim31, wherein said transmit coils are orthogonal to all other transmitcoils within the sensor.
 33. The method of claim 31, wherein said sensorcomprises three individual transmit coils, and wherein the emissionsgenerated by each of said individual transmit coils are separated by 120degrees of phase.
 34. The method of claim 31, wherein measuring thephase of signals received by said receiver coil is performed by saidtransponder.
 35. The method of claim 31, wherein said radio frequencyemissions from each of said transmit coils of each of said at least twosensors are emitted continuously.
 36. The method of claim 35,additionally comprising: determining when said transponder is in motion;and processing said radio frequency emissions by said in motiontransponder so as to determine the relative position of said in motiontransponder with respect to said at least two sensors.
 37. The method ofclaim 35, additionally comprising: determining when said transponder isbeing tampered with; and processing said radio frequency emissions bysaid tampered transponder so as to determine the relative position ofsaid tampered transponder with respect to said at least two sensors. 38.A radio frequency positioning system, comprising: a radio frequencyidentification (RFID) transponder having an antenna coil configured toreceive radio frequency signals; a first sensor including at least oneantenna coil, said first sensor capable of transmitting a first signalhaving a first phase from said at least one antenna coil of said firstsensor; a second sensor including at least one antenna coil, said secondsensor capable of transmitting a second signal having a second phasefrom said at least one antenna coil of said second sensor; and means fordetermining the position of said transponder relative to said first andsecond sensors based on the phase relationship between said first andsecond signals received by said antenna coil of said transponder. 39.The system of claim 38, wherein said second sensor is at a differentlocation than said first sensor.
 40. The system of claim 38, whereinsaid transponder is in a predetermined spatial relationship with respectto said first and second sensors.
 41. The system of claim 38, whereinone of said sensors is further capable of transmitting signals ofdiffering phase on each of a plurality of individual antenna coils. 42.The system of claim 41, wherein said at least one antenna coils of saidfirst and second sensors are orthogonal to all other antenna coilswithin the same sensor.
 43. The system of claim 41, wherein each of saidsensors comprises three individual antenna coils, and wherein thesignals transmitted by each of said individual antenna coils areseparated by 120 degrees of electrical phase.
 44. The system of claim38, wherein the means for determining the position of said transpondercomprises means for measuring the relative intensity of each of saidfirst and second signals.
 45. The system of claim 38, wherein theposition of said transponder relative to said first and second sensorsis determined within said transponder.
 46. The system of claim 38,wherein said first signal and said second signal are continuouslytransmitted.
 47. The system of claim 46, wherein said transponderincludes a motion sensor operable to determine when said transponder isin motion, such that when it is determined that said transponder is inmotion, said first and second signals are processed by said in motiontransponder so as to determine the relative position of said in motiontransponder with respect to said first and second sensors.
 48. Thesystem of claim 46, wherein said transponder includes a tamper detectoroperable to determine when said transponder is being tampered with, suchthat when it is determined that said tampered transponder is beingtampered with, said first and second signals are processed by saidtampered transponder so as to determine the relative position of saidtampered transponder with respect to said first and second sensors. 49.A positioning system, including: an identification transponder having atleast one antenna disposed therein, said at least one antenna configuredto receive communication signals; a first coil oriented in a firstdirection relative to a reference point; and a second coil oriented in asecond direction relative to said reference point, said second coilbeing in a different physical location than said first coil, said firstcoil and said second coil being configured to continuously transmit afirst and a second communication signal, respectively, wherein saidfirst and second communication signals transmitted by said first andsecond coils are different in phase; wherein said transponder includes amotion sensor operable to determine when said transponder is in motion,and wherein in response to said determination that said transponder isin motion, said transponder uses said phase difference to determine aposition of said transponder relative to said first and second coils.50. The positioning system of claim 49, wherein said transponderoperates in the radio frequency (RF) domain.
 51. The positioning systemof claim 49, further including a tamper detector operable to determinewhen said transponder is being tampered with, wherein in response tosaid determination that said transponder is being tampered with, saidtransponder determines a position of said transponder relative to saidfirst and second coils based upon said phase difference.